The invention relates to a method for identifying a portable transponder as being located in the near area of a transmitting antenna of a base station, and to an antitheft system for a motor vehicle.
One problem that occurs with antitheft systems for motor vehicles which operate by means of wire-free communication between a base station fixed to the vehicle and a portable transponder, is as follows:
The transmission power of the base station and of the transponder are generally fixed such that signals from the base station are received by the transponder only in a near area, that is to say in the immediate transmission area of the transmitting antenna of the base station and, in addition, signals from the transponder are received by the base station only when the transponder is located in the near area. By manipulation, it is possible to eavesdrop on the communication path between the base station and the transponder and to reproduce the signals from a long distance.